How can I find out if my phone is jail broken

There is no easy way to tell. However, to jailbreak it someone who knows how must have physical possession of the phone while it was unlocked for 15-30 minutes. If this is a possibility you need to restore the phone to factory settings and not restore any backup to it; in other words, start over.

See—>Restore your iPhone, iPad, or iPod to factory settings - Apple Support

Most of the time there is no way to tell, however sometimes a tell tale sign is the presence of an application called Cydia, or sometimes unexpected profiles on the device. However the easiest way to tell is, did you let someone else use your device? If you didn't, and didn't actively seek out a jailbreak, then you are fine. The device can not be jailbroken without physical access to the device.

Some apps like capital one once you open them if your device is jailbroken it will give you a notification saying so

andreag83 wrote:

what to know if iPhone is jail broken

If you updated to iOS 14 and a bank app is telling you that, it is likely the bank app needs to be updated for iOS 14.

It's a poorly worded error message.